The present invention relates to an optical disk having a super-resolution film.
In recent years, an optical disk, particularly, a digital video disk (DVD), attracts attentions as a recording medium indispensable for information industry, and the market of the optical disk is being enlarged. Such an optical disk, which is a non-contact type, has a high recording density. However, since the capacity of software and program of games and the like is expected to increase without fail, a further improvement in the recording density is required in the optical disk. The most effective method for increasing the density of the optical disk is to diminish a convergence area of laser beam.
One of the methods for diminishing the convergence area of the laser beam is to decrease a wavelength of laser beam. It should be noted in this respect that change from a GaAlInP-based semiconductor laser having a wavelength of 650 nm to a GaN-based semiconductor laser having a wavelength of 400 nm is being made nowadays. In view of the characteristics of GaN, it may be highly reasonable that the wavelength of the semiconductor laser will be further decreased to 350 nm in future.
A super-resolution is considered to provide another method for diminishing the convergence area of laser beam. The concept of the super-resolution will now be described with reference to FIG. 1. Specifically, the optical disk 1 shown in FIG. 1 comprises the recording layer 2 and the super-resolution film 3. Data are recorded in recorded portions 2a. The recording layer 2 is irradiated with a laser beam 4 through the super-resolution film 3. The light intensity in the central portion of the laser beam spot on the super-resolution film 3 is higher than that in the peripheral portion. It should be noted that the super-resolution film 3 permits selectively transmitting the high intensity portion of the laser beam 4, with the result that an aperture smaller than the diffraction limit of the laser beam is formed in the super-resolution film 3. It follows that the recording layer 2 is irradiated with the laser beam 4 having a spot size smaller than the diffraction limit, making it possible to read out the data even if the data are recorded in a high density. In addition, a carrier-to-noise ratio (CNR) for readout from short pits may be improved.
The super-resolution films include a heat-mode type functions through heat and a photon-mode type functions depending only on light power. The heat-mode type super-resolution film varies its transmittance at a high temperature portion in the center of the beam spot with transformation of the film material through melting or composition change. As the photon-mode type super-resolution film, the most promising material is that represents absorption saturation. The absorption saturation phenomenon denotes the phenomenon that, in the case of a high light intensity, the absorption of the super-resolution film is decreased so as to increase the transmittance. This phenomenon is generated under the situation that, in a super-resolution film irradiated with light, electrons are excited to an upper level so as to change light absorption characteristics. To be more specific, when the electrons excited upon irradiation with light are subjected to harmonic oscillation, anharmonic oscillation is induced so as to change the light absorption characteristics and the refractive index of the super-resolution film.
In order to improve the super-resolution characteristics of the super-resolution film utilizing the absorption saturation phenomenon described above, it has been clarified that the material contained in the super-resolution film should desirably exhibit large third-order nonlinear optical characteristics. It has been found that the material having a large third-order nonlinear optical constant includes, for example, a material utilizing π-conjugated or σ-conjugated electrons, a material utilizing metal plasmon, and a material utilizing a semiconductor exciton. Particularly, a material utilizing a semiconductor exciton is being studied extensively.
It is known to the art that, if the dimension of a material utilizing the semiconductor exciton is lowered, it is possible to stabilize the harmonic oscillation of the excited electrons so as to enlarge the anharmonic oscillation, with the result that the third-order nonlinear optical characteristics are enhanced and state density is increased. The low dimensional system includes a super-lattice (two-dimensional), a quantum wire (one-dimensional), and quantum dots formed of nanocrystallites (zero-dimensional). Particularly, it is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 06-28713 and Japanese Patent Disclosure (Kokai) No. 11-86342 that the semiconductor nanocrystallites are desirable for forming a super-resolution film. The super-resolution films disclosed in these patent specifications are prepared by spin-coating of a dispersion of the semiconductor nanocrystallites and a transparent matrix in a solvent, or by sputtering the semiconductor nanocrystallites.
However, it is desired to further improve the super-resolution characteristics even in a super-resolution film utilizing the semiconductor nanocrystallites so as to increase the recording density of the optical disk.